The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should be construed as an admission that such art is widely known or forms part of common general knowledge in the field.
Fesoterodine fumarate is the international non-proprietary name (INN) of the active ingredient isobutyric acid 2-[(R)-3-diisopropylammonium-1-phenylpropyl)-4-(hydroxy-methyl)phenyl ester hydrogen fumarate, which can be represented by the structural formula (1)

Fesoterodine fumarate is approved in Europe and USA for the treatment of overactive bladder syndrome with the commercial name TOVIAZ®.
U.S. Pat. No. 6,713,464 B1 discloses a variety of 3,3-diphenylpropylamine derivatives, processes for their preparation, pharmaceutical compositions comprising the derivatives, and methods of use thereof. These compounds are anti-muscarinic agents with superior pharmacokinetic properties compared to existing drugs such as oxybutynin and tolterodine which are useful in the treatment of urinary incontinence, gastrointestinal hyperactivity (irritable bowel syndrome) and other smooth muscle contractile conditions.
Improved processes for the preparation of fesoterodine and its pharmaceutically acceptable salts are disclosed in US Patent application 2009/0306384 A1, and International (PCT) Publication No. WO 2009/037569 A2.
U.S. Patent Application No. 2010/0168459 A1 dislcoses the use of N,N-diisopropylethyl amine for the condensation of diol compound and isobutyryl chloride for the ultimate formation Fesoterodine free base.
U.S. Patent Application No. 2010/174107 A1 discloses use of Turbo Grignard reagent of formula R1(MgX)n.LiY, wherein R1 is an aromatic, aliphatic, carbocyclic or heterocyclic organic group having 1 to 24 carbon atoms; X and Y are independently selected from Cl, Br and I and n is 1 or 2, for the formation of protected ester from the bromo protected compound as shown in Scheme-1 (wherein simple Grignard reagent is used).
International (PCT) Publication No. WO 2009/044278 A1 of relates to amorphous Fesoterodine fumarate being characterized by XRPD & IR.
International (PCT) Publication No. WO 2009/122303 A1 discloses preparation of Fesoterodine mandelate salt.
International (PCT) Publication No. WO 2010/010464 A2 discloses an impurity of fesoterodine i.e., fesoterodine dehydroxy impurity.
US patent application No. 2010/0152483 A1 discloses Fesoterodine fumarate in crystalline Form—I being characterized by XRPD, DSC, IR, 13C NMR and Raman spectra. US Patent application also discloses Amorphous Fesoterodine Fumarate being characterized by XRPD and IR.
U.S. Pat. No. 6,858,650 describes various acid addition salts of 3,3-dipbenylpropylamine derivatives.
The references cited in the above art for the preparation of Fesoterodine fumarate (I) involves reaction conditions, which leads to formation of side reactions and byproducts. Processes disclosed in the art involve the formation of intermediates as an oil or syrup with low yield and purity.
PCT application WO 2010010464 A2 discloses the process for obtaining fesoterodine fumarate having a 90 volume-percent of the particles (D90) with a size of less than or equal to about 200 microns and process for achieving the particle size (D90) with a size of less than or equal to about 200 microns.
It is reported in the literature that fesoterodine may exhibit substantial degradation in a humid environment and at increased temperature. It is believed that hydrolyzation and oxidation are among the major mechanisms resulting in degradation. Therefore, it is desired to develop a fesoterodine fumarate, which is stable under stress condition. It has been found, surprisingly, that fesoterodine fumarate with higher particle size significantly slow down the degradation of fesoterodine under stress conditions.